Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a boil. Add the broccoli florets and cook for 3-4 minutes, or until bright green and crisp-tender. Drain immediately and plunge into an ice bath to stop the cooking and preserve the vibrant color. Once cooled, drain thoroughly and pat dry. Set aside.

In a large saucepan or Dutch oven, melt 1/4 cup (1/2 stick) of unsalted butter over medium heat. Once melted, add the all-purpose flour and whisk constantly for 1-2 minutes to create a smooth roux. It should be light golden and smell slightly nutty.

Gradually pour in the whole milk, whisking continuously to prevent lumps. Continue to whisk until the sauce thickens, about 5-7 minutes. Once thickened, reduce the heat to low. Stir in the salt, black pepper, garlic powder, and onion powder. Add 3 cups of the shredded sharp cheddar cheese, stirring until completely melted and smooth. Remove from heat.

Gently fold the drained broccoli florets into the cheese sauce until evenly coated. Pour the mixture into a 9x13-inch baking dish, spreading it out evenly.

In a small bowl, melt the remaining 1/4 cup (1/2 stick) of unsalted butter. Add the panko breadcrumbs and grated Parmesan cheese, stirring until the breadcrumbs are lightly coated with butter.

Sprinkle the buttered breadcrumb mixture evenly over the top of the casserole. Bake for 20-25 minutes, or until the casserole is bubbly around the edges and the topping is golden brown and crispy.

Remove from the oven and let rest for 5-10 minutes before serving. This allows the sauce to set slightly.